In recent years, optical coherence tomography (OCT) has been drawing attention. The OCT creates an image representing the exterior or interior structure of an object to be measured using light beams from a laser light source or the like. Unlike X-ray computed tomography (CT), the OCT is not invasive on the human body, and therefore is expected to be applied to the medical field and the biological field, in particular. For example, in the ophthalmological field, apparatuses for forming images of the fundus oculi or the cornea have been in practical use. Such an apparatus using OCT imaging (OCT apparatus) can be used to observe a variety of sites of a subject's eye. In addition, because of the ability to acquire high precision images, the OCT apparatus is applied to the diagnosis of various eye diseases.
Among the OCT apparatuses, regarding those that use Fourier domain OCT imaging, it is known that a fixed pattern noise (FPN) is present in corrected data, and that the FPN may not be completely removed and appear in images, resulting in a reduction in image quality.
Regarding apparatuses that use spectral domain OCT imaging (hereinafter, SD-OCT), FPN can be removed, for example, by calculating the average spectrum in the A-line direction in each irradiation position and subtracting the average spectrum from spectrums measured.
Meanwhile, apparatuses that use swept-source OCT imaging (hereinafter, SS-OCT) are not capable of removing FPN even with the same method as the SD-OCT. Considered as a factor of this is jitter in the time-axis direction between the timing of controlling the light source and the timing of light emission from the light source. Due to the influence of jitter, SS-OCT is considered as unsuitable for imaging with the use of phase information (such as Doppler OCT, Phase Variance OCT, and the like) as compared to SD-OCT.
As for a method to reduce the influence of jitter in SS-OCT, reference may be had to Meng-Tsan Tsai et al, “Microvascular Imaging Using Swept-Source Optical Coherence Tomography with Single-Channel Acquisition” Applied Physics Express 4 (2011), pp. 097001-1 to 097001-3, and WooJhon Choi et al., “Phase-sensitive swept-source optical coherence tomography imaging of the human retina with a vertical cavity surface-emitting laser light source” Optics Letters, vol. 38, No. 3, 2013 Feb. 1, pp. 338-340. These documents disclose a method of removing FPN. According to the method, trigger signals are generated by fiber Bragg grating (FBG), and an image is formed after the phases of interference signals are adjusted with reference to the trigger signals.
However, in this method, the phases of interference signals are adjusted in response to wavenumber clocks for sampling the interference signals besides the trigger signals to thereby reduce the influence of jitter. This leads to the complication of the configuration and control of the apparatus.